Implantable medical devices are used to deliver therapy to patients suffering from a variety of conditions. Examples of implantable medical devices are implantable pacemakers and implantable cardioverter-defibrillators (ICDs), which are electronic medical devices that monitor the electrical activity of the heart and provide electrical stimulation to one or more of the heart chambers as needed. For example, a pacemaker senses an arrhythmia, i.e., a disturbance in heart rhythm, and provides appropriate electrical stimulation pulses, at a controlled rate, to selected chambers of the heart in order to correct the arrhythmia and restore the proper heart rhythm. The types of arrhythmias that may be detected and corrected by pacemakers include bradycardias, which are unusually slow heart rates, and certain tachycardias, which are unusually fast heart rates.
Implantable cardioverter-defibrillators (ICDs) also detect arrhythmias and provide appropriate electrical stimulation pulses to selected chambers of the heart to correct an abnormal heart rate or rhythm. In contrast to pacemakers, however, an ICD can deliver cardioversion and high energy defibrillation pulses that are much stronger than typical pacing pulses. This is because ICDs are generally designed to correct fibrillation and tachycardia episodes. To correct such arrhythmias, an ICD delivers a low, moderate, or high-energy therapy.
Modern pacemakers and ICDs are designed with ergonomic shapes that are relatively compliant with a patient's implant location and tend to minimize patient discomfort. As a result, the corners and edges of the devices are typically designed with relatively generous radii to present a device having smoothly contoured exterior surfaces. It is also desirable to minimize the volume occupied by the devices as well as their mass to further limit patient discomfort.
The electrical energy for the therapy delivered by an ICD is generated by delivering electrical current from a power source (battery) to charge capacitors to store energy. The capacitors are capable of rapidly discharging under computer control to deliver one or more appropriate waveforms that deliver energy via electrodes disposed in communication with a patient's heart. In order to provide timely therapy to the patient after the detection of ventricular fibrillation, for example, it is necessary to charge the capacitors with the required amount of energy as quickly as possible. Thus, the battery in an ICD must have a high rate capability to provide the necessary current to charge the capacitors. In addition, since ICDs are implanted in patients, the battery must be able to accommodate physical constraints on size and shape.
Batteries or cells are volumetrically constrained systems. The size or volume of components that go into a battery (cathode, anode, separator, current collectors, electrolyte, etc.) cannot exceed the available volume of the battery case. The arrangement of the components affects the amount or density of active electrode material contained within the battery case.
One battery suitable for use in ICDs is disclosed in U.S. Pat. No. 4,830,940 to Keister et al, which patent is incorporated herein by reference. As disclosed therein, the anode material of the battery comprises lithium and the reactive cathode material comprises silver vanadium oxide. The anode is constructed in a serpentine-like configuration with cathode plates inserted between each of the convolutions thereof on both sides thereof. The electrolyte for a lithium battery or cell is a liquid organic type which comprises a lithium salt and an organic solvent. Both the anode and the cathode plates are encapsulated in an electrically insulative separator material. An improvement to this design is disclosed in U.S. Pat. No. 5,147,737 to Post et al., in which the active material on the serpentine-type electrode is positioned so that the sections of the serpentine-like structure which do not face cathode plates do not contain anode active material.
Known high current power sources used in ICDs employ deep, prismatic, six-sided rectangular solid shapes in packaging of the electrode assemblies. Examples of such deep package shapes can be found in, e.g., U.S. Pat. No. 5,486,215 to Kelm et al., and U.S. Pat. No. 6,040,082 to Haas et. al. These prismatic cases have proven effective for housing and electrically insulating the electrode assemblies.
Conventional lithium batteries can also employ an electrode configuration sometimes referred to as the “jelly roll” design, in which the anode, cathode and separator elements are overlaid and coiled up in a spiral wound form. A strip sheet of lithium or lithium alloy comprises the anode, a cathode material supported on a charge collecting metal screen comprises the cathode, and a sheet of non-woven material separates the anode and cathode elements. These elements are combined and wound to form a spiral. Typically, the battery configuration for such a wound electrode would be cylindrical. An advantage of this design is that there need not be anode material which is not mated to cathode material in the jelly roll electrode configuration. Such designs therefore have the potential for an improved match between the cathode and anode components and improved uniformity of anode and cathode utilization during discharge.
It has also been known to adapt wound electrodes to a prismatic case configuration by departing from a true spiral winding. For example, U.S. Pat. No. 2,928,888 discloses in FIGS. 5a and 5b therein an oblong electrode assembly wound on an elongated mandrel for use in a rectangular case. Also, U.S. Pat. No. 4,051,304 discloses in FIG. 2 therein another oblong wound electrode assembly for use in a rectangular case.
U.S. Pat. No. 4,761,352 to Bakos et al. discloses yet another electrode assembly design comprising an accordion folded electrode assembly with unitary members for both the anode and cathode electrode strips. The cathode strip is approximately half the length of the anode strip and the anode strip is folded over the cathode strip to “sandwich” the cathode between two layers of the anode. The resulting form is then manually folded in an alternating series of “V” folds (best shown in FIG. 4 of the '352 patent).
There exists a need for a battery for implantable medical devices which optimizes volumetric efficiency while allowing for flexibility in designing the shape of the battery to match the contours of an implantable medical device and to fit within the available device space.